Control Channel Design for Many-Antenna MU-MIMO Systems

ABSTRACT

Disclosed embodiments include methods for control channel design in many-antenna multi-user (MU) multiple-input multiple-output (MIMO) wireless systems. A beacon comprising an identifier of a many-antenna base station is encoded into a base sequence. A plurality of synchronization sequences is generated based on the encoded base sequence and a set of orthogonal beam sequences. The many-antenna base-station transmits, using a plurality of antennas, the plurality of synchronization sequences in a plurality of beam directions associated with the set of orthogonal beam sequences for synchronization and associated with users without knowledge of channel state information (CSI).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/152,675, filed Apr. 24, 2015, which is hereby incorporatedby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made in part with government support under grantnumbers CNS0751173, CNS0923479, CNS1012831, CNS1126478, and CNS1218700awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

This disclosure generally relates to a method and apparatus for wirelesscommunications, and more particularly relates to a control channeldesign for many-antenna multi-user (MU) multiple-input multiple-output(MIMO) systems.

Many-antenna MU-MIMO based communication faces a previously unaddressedchallenge that it lacks a practical control channel. The potential rangeof MU-MIMO beamforming systems scales with up to the square of a numberof antennas at a base station once the base station has channel stateinformation (CSI). On the other hand, the range of traditional controlchannel operations remains constant since the control channel operationstake place before or during CSI acquisition. The range gap betweenno-CSI and CSI modes presents a challenge to the efficiency andfeasibility of many-antenna base stations.

Many-antenna MU-MIMO based communication represents a rapidly growingresearch field, which has recently shown promise of commercialization.However, there are still certain system challenges facing the creationof practical many-antenna base stations for many-antenna MU-MIMOwireless systems. One issue in current architectures is the lack of anefficient and reliable control channel that is required for variousnetwork operations. Wireless communication systems typically realizeoperations on the control channel using a single high-power antenna, orsimple diversity schemes. However, these methods rapidly become veryinefficient as the number of base-station antennas increases.

SUMMARY

Disclosed embodiments include a method for open-loop control operationsperformed by a serving many-antenna base station (BS). The method foropen-loop control starts by encoding a beacon with an identified (ID) ofthe BS into a base sequence. The many-antenna BS generates a pluralityof synchronization sequences by spreading the encoded base sequence witha set of orthogonal beam sequences. The many-antenna BS transmits, usinga plurality of antennas, the plurality of synchronization sequences in aplurality of different beam directions determined by the orthogonal beamsequences, thus facilitating synchronization and association (andpossibly other control operations) of users served by the many-antennaBS without any users' information at the BS.

Disclosed embodiments include a method for open-loop control operationsperformed by a user equipment (UE) served by the many-antenna BS. Themethod for open-loop control starts by receiving the plurality ofsynchronization sequences having different signal strengths andtransmitted in different beam directions from the many-antenna BS. UEcan utilize one of the received synchronization sequences to achievetime and frequency synchronization with the many-antenna BS. Aftersynchronizing with the many-antenna BS, UE can decode, from the receivedsynchronization sequence, a beacon with an identifier (ID) of the BS andperforms an association procedure with the BS. UE can also receivesynchronization sequences from one or more other BSs in the neighborhoodand perform synchronization/association with any of these BSs if theassociation with the original many-antenna BS is not fully completed.After performing synchronization and association with the BS, UE canalso page the serving BS and request random access from the serving BS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example many-antenna multi-user multiple-input multipleoutput (MU-MIMO) wireless communication system, in accordance withembodiments of the present disclosure.

FIG. 2 is an example many-antenna base station operating in differentmodes, in accordance with embodiments of the present disclosure.

FIG. 3 is an example table showing analytical results of gains and gaingaps for different modes of operation of the many-antenna base stationillustrated in FIG. 2, in accordance with embodiments of the presentdisclosure.

FIG. 4 is an example many-antenna base station that may performopen-loop beamforming, in accordance with embodiments of the presentdisclosure.

FIG. 5 is an example many-antenna base station that may performopen-loop beamforming with applied additional coding gain, in accordancewith embodiments of the present disclosure.

FIGS. 6A, 6B, 6C, and 6D are examples of different frame structures fordifferent operations of a many-antenna base station in a many-antennaMU-MIMO wireless communication system, in accordance with embodiments ofthe present disclosure.

FIG. 7 is an example many-antenna base station transmitting asynchronization sequence (e.g., beacon) by employing beamsweeping andcoding, in accordance with embodiments of the present disclosure.

FIG. 8 is an example many-antenna base station that simultaneouslyperforms synchronization and paging of users, in accordance withembodiments of the present disclosure.

FIG. 9 is an example table that shows analysis of a control channeloverhead for a many-antenna MU-MIMO wireless communication system, inaccordance with embodiments of the present disclosure.

FIG. 10 is an example graph illustrating beacon detection performancefor synchronization and association of users in a many-antenna MU-MIMOwireless communication system, in accordance with embodiments of thepresent disclosure.

FIG. 11 is an example graph illustrating beacon detection performancefor synchronization and association of users in a many-antenna MU-MIMOwireless communication system versus an uplink signal strength, inaccordance with embodiments of the present disclosure.

FIG. 12 is an example graph illustrating cumulative distributionfunctions of paging delay in a many-antenna MU-MIMO wirelesscommunication system, in accordance with embodiments of the presentdisclosure.

FIG. 13 is an example graph illustrating cumulative distributionfunctions of a carrier frequency offset (CFO) estimation error in amany-antenna MU-MIMO wireless communication system, in accordance withembodiments of the present disclosure.

FIG. 14 is a block diagram of an example wireless device, in accordancewith embodiments of the present disclosure.

FIG. 15 is flow chart illustrating a method that may be performed at amany-antenna base station of a many-antenna MU-MIMO wirelesscommunication system, in accordance with embodiments of the presentdisclosure.

FIG. 16 is a flow chart illustrating a method that may be performed at auser equipment (UE) in communication with a many-antenna base station ofa many-antenna MU-MIMO wireless communication system, in accordance withembodiments of the present disclosure.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication systems, including communication systems that are based onan orthogonal multiplexing scheme. Examples of such communicationsystems include Spatial Division Multiple Access (SDMA), Time DivisionMultiple Access (TDMA), Orthogonal Frequency Division Multiple Access(OFDMA) systems, Single-Carrier Frequency Division Multiple Access(SC-FDMA) systems, and so forth. An SDMA system may utilize sufficientlydifferent directions to simultaneously transmit data belonging tomultiple user terminals. A TDMA system may allow multiple user terminalsto share the same frequency channel by dividing the transmission signalinto different time slots, each time slot being assigned to differentuser terminal. An OFDMA system utilizes orthogonal frequency divisionmultiplexing (OFDM), which is a modulation technique that partitions theoverall system bandwidth into multiple orthogonal sub-carriers. Thesesub-carriers may also be called tones, bins, etc. With OFDM, eachsub-carrier may be independently modulated with data. An SC-FDMA systemmay utilize interleaved FDMA (TDMA) to transmit on sub-carriers that aredistributed across the system bandwidth, localized FDMA (LFDMA) totransmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA)to transmit on multiple blocks of adjacent sub-carriers. In general,modulation symbols are created in the frequency domain with OFDM and inthe time domain with SC-FDMA.

The teachings herein may be incorporated into (e.g., implemented withinor performed by) a variety of wired or wireless apparatuses (e.g.,nodes). In some embodiments, a node comprises a wireless node. Suchwireless node may provide, for example, connectivity for or to a network(e.g., a wide area network such as the Internet or a cellular network)via a wired or wireless communication link. In some embodiments, awireless node implemented in accordance with the teachings herein maycomprise an access point or an access terminal.

An access point (“AP”) may comprise, be implemented as, or known asNodeB, Radio Network Controller (“RNC”), eNodeB, Base Station Controller(“BSC”), Base Transceiver Station (“BTS”), Base Station (“BS”),Transceiver Function (“TF”), Radio Router, Radio Transceiver, BasicService Set (“BSS”), Extended Service Set (“ESS”), Radio Base Station(“RBS”), or some other terminology. In some implementations, an accesspoint may comprise a set top box kiosk, a media center, or any othersuitable device that is configured to communicate via a wireless orwired medium. According to certain embodiments of the presentdisclosure, the access point may operate in accordance with theInstitute of Electrical and Electronics Engineers (IEEE) 802.11 familyof wireless communications standards.

An access terminal (“AT”) may comprise, be implemented as, or known asan access terminal, a subscriber station, a subscriber unit, a mobilestation, a remote station, a remote terminal, a user terminal, a useragent, a user device, user equipment, a user station, or some otherterminology. In some implementations, an access terminal may comprise acellular telephone, a cordless telephone, a Session Initiation Protocol(“SIP”) phone, a wireless local loop (“WLL”) station, a personal digitalassistant (“PDA”), a handheld device having wireless connectioncapability, a Station (“STA”), or some other suitable processing deviceconnected to a wireless modem. Accordingly, one or more aspects taughtherein may be incorporated into a phone (e.g., a cellular phone or smartphone), a computer (e.g., a laptop), a portable communication device, aportable computing device (e.g., a personal data assistant), a tablet,an entertainment device (e.g., a music or video device, or a satelliteradio), a television display, a flip-cam, a security video camera, adigital video recorder (DVR), a global positioning system device, or anyother suitable device that is configured to communicate via a wirelessor wired medium. According to certain embodiments of the presentdisclosure, the access terminal may operate in accordance with the IEEE802.11 family of wireless communications standards.

A multiple-input multiple-output (MIMO) base station (or access point)can have two modes of operation based on its knowledge of users' channelstate information (CSI), i.e., the no-C SI mode occurring before thebase station has the CSI knowledge for supported active users, and theCSI mode that utilizes a more efficient MIMO communication link betweenthe base station and supported active users generated based on CSIcollected at the base station. To collect CSI, the base stationestablishes time-frequency synchronization with supported users (oraccess terminals), and then receives uplink pilots back from thesynchronized users. Furthermore, once a user becomes inactive, the basestation can be configured to notify inactive user of an incomingtransmission, i.e., the base station can page the inactive user,prompting the inactive user to send a pilot. All of these operations arepart of a control channel, which is traditionally sent entirely duringthe no-CSI mode.

In MIMO wireless communication systems, the CSI mode has a gain of up toM² higher than the no-CSI mode, where is a number of antennas at a basestation. When M is small, as in current systems, one can overcome thisgain gap by using a lower modulation rate or a coding gain in the no-CSImode. However, as M increases, the gap between the CSI mode and theno-CSI mode quickly becomes large. In existing systems, all controlchannel operations are performed in the no-C SI mode and communicatedomni-directionally to the entire coverage area. Thus, the base station'soperational range can be limited by the no-CSI mode, which issignificantly shorter than that of the CSI mode. One naive solution canbe to employ a higher transmission power in the no-CSI mode incomparison with the CSI mode. However, this approach leads to a moreexpensive hardware (e.g., power amplifier at the base station withhigher power consumption) and increased inter-cell interference.

Described embodiments include methods for control channel design thataddress the aforementioned gain gap for base stations (or access points)with a large number of antennas (e.g., many-antenna base stations).There are two key insights that are leveraged in the present disclosure.The first insight is that as much of a control channel as possibleshould be sent over the CSI mode. In accordance with embodiments of thepresent disclosure, control channel operations that utilize the no-CSImode of a many-antenna base station are time-frequency synchronization,association, CSI collection, paging, and random access, which representoperations that are required to establish the CSI mode. By implementingthe remaining control channel operations over the CSI mode, efficiencyof the remaining control channel operations can be substantiallyincreased and the aforementioned gain gap can be avoided. The secondinsight applied for control channel design in the present disclosure isthat synchronization and association are not time-critical controlchannel operations. For example, synchronization can be valid forhundreds of ms, whereas association is performed only once. Thus, byreducing a frequency of performing synchronization operation, a channeloverhead in the no-CSI mode associated with synchronization andassociation operations can be substantially reduced, at the cost ofslightly increased association latency at cell edges.

Described embodiments include methods for open-loop beamforming andapplying coding gains to ensure that many-antenna base stations canachieve their full potential range even in the no-CSI mode of operation.Through open-loop beamforming, control channel design presented in thisdisclosure is able to utilize the full diversity, power, and beamforminggains from all of antennas at a many-antenna base station, enabling thepotential range to scale with a number of base station antennas (e.g.,by a factor of M). Because there is a certain gap between the potentialrange of open-loop beamforming and the potential range of its MU-MIMOcounterpart—closed-loop beamforming, coding gains can be employed in thepresent disclosure to further increase the potential range and to ensurethat synchronization and paging are reliable even at cell edges. To beas efficient as possible, a many-antenna MU-MIMO wireless communicationsystem that utilizes the control channel design presented hereinperforms only the aforementioned essential tasks and communicationsoutside of the CSI mode, which offers much higher spectral capacity.

For some embodiments, a many-antenna base station of the MU-MIMOwireless communication system presented in this disclosure utilizesopen-loop beamforming over the control channel in the no-CSI mode tosweep extra-long synchronization sequences across a coverage area. Thesynchronization sequences employed herein may enable users to establishtime-frequency synchronization with the many-antenna base station, andmay also encode the base-station identification (ID) for performingassociation. In one or more embodiments, the synchronization sequencestransmitted from the many-antenna base station may further encode userIDs for performing simultaneous synchronization/association and paging.

For some embodiments, certain communication parameters may bedynamically configured, such as beam patterns, a sweep rate, and asynchronization sequence length to match a required gain for fullcoverage of a desired area. Furthermore, by increasing open-loopbeamforming and coding gains in the no-CSI mode while reducing themodulation rate and/or number of users served in the CSI mode, thecontrol channel design presented herein can be used to extend the rangeof the many-antenna base station in remote areas.

In accordance with illustrative embodiments, a many-antenna base stationof an MU-MIMO wireless communication system that employs control channeldesign presented herein may communicate with users over a 2.4 GHzcommunication link using an array of 108 antennas to evaluateperformance and control channel overhead. Conducted measurementspresented in detail below show that the presented control channel designprovides over a 40 dB gain compared to traditional control channeloperations. As discussed in more detail below, this gain enablesreliable synchronization to mobile users at over 250 meters while usingless than 100 μW of transmission power per base station antenna, orapproximately 10 mW of total base station transmission power, employingonly standard low-gain 3 dBi omnidirectional antennas at themany-antenna base station. The presented design of control channelfacilitates collecting high resolution channel measurements in highlymobile environments, with less than 0.5% channel overhead. To reduce theoverhead of paging delay, a paging scheme is employed that leveragesuser's last known location for directing a paging signal.

FIG. 1 illustrates an example many-antenna MU-MIMO wirelesscommunication system 100, in accordance with embodiments of the presentdisclosure. As illustrated in FIG. 1, a many-antenna base station 102may comprise an array of large number of antennas (e.g., up to 108antennas as described in the illustrative embodiment). The many-antennabase station 102 may employ the antenna array to communicate with aplurality of mobile users 104. As illustrated in FIG. 1, if users'synchronization is performed in traditional manner employing the no-CSImode, a coverage range (i.e., gain) may be limited to a region 106(e.g., cumulative gain for all users). The region (i.e., gain) 106 issubstantially smaller than a region (i.e., gain) 108 that represents acoverage range of MU-MIMO communication in the CSI mode (e.g.,cumulative gain for all users). As further illustrated in FIG. 1,closed-loop beamforming can be applied for MU-MIMO communication in theCSI mode for directing signal energy within a certain direction (e.g.,beam 110) toward each user 104 when CSI related to that user 104 isknown at the many-antenna base station 102.

For some embodiments, as discussed in more detail below, by applyingopen-loop beamforming in the no-CSI mode for synchronizing each user 104with the many-antenna base station 102, a coverage gap (i.e., gain gap)between traditional no-CSI communication (e.g., region 106) and MU-MIMOcommunication (e.g., region 108) may be substantially reduced. Thecoverage region (i.e., gain) of the no-CSI mode can be further extendedby applying coding gain, as also discussed in more detail below.

Beamforming and MU-MIMO

As illustrated in FIG. 1, beamforming may utilize multiple antennas ofthe base station 102 transmitting to the users 104 at the same frequencyto realize directional transmission, i.e., transmission within regionsof space (e.g., beams) 110. Constructive and destructive interference ofsignals from multiple antennas of the base station 102 may cause asignal strength received at a user 104 to vary spatially, leading to abeam pattern 110. The beam pattern 110 can be altered by changingbeamforming weights applied to each antenna of the base station 102,effectively altering the amplitude and phase of the signal sent fromthat base station antenna.

In some embodiments, a many-antenna base station (e.g., the many-antennabase station 102 illustrated in FIG. 1) may employ open-loop beamformingfor users' synchronization in the no-CSI mode. In this case, themany-antenna base station may utilize pre-computed beamforming weights(e.g., beamweights), such as Discrete Fourier Transform (DFT) basedbeamforming weights or Hadamard-based beamforming weights, to steer abeam in a desired spatial direction, without knowledge of the users'locations. On the other hand, the closed-loop or adaptive beamformingemploys known CSI between the many-antenna base station and intendedusers to calculate the beamweights that maximize the signal strength atthe intended users and minimize the interference of unintended users. Inthe case of closed-loop (adaptive) beamforming, the intended users mayprovide (e.g., via feedback channels) information about their locationsand/or pilots to the many-antenna base station. Based on the providedusers' locations and/or pilots, the many-antenna base station canestimate CSI related to the intended users and shape/steer beams of datatowards the intended users.

The many-antenna base station 102 may utilize multiple antennas forserving multiple users simultaneously on the same time-frequency-coderesources, typically through closed-loop beamforming. Each base stationantenna may comprise its own radio (not shown in FIG. 1). Forsimplicity, the term antenna is used in the present disclosure toinclude both the radio and antenna. The spectral and energy efficienciesof MU-MIMO wireless communication systems (e.g., the MU-MIMO wirelesscommunication system 100 illustrated in FIG. 1) grow with the number ofbase-station antennas (e.g., by the factor of M) and the number ofconcurrent users (e.g., by the factor of K), wherein M≥K. Hence,implementation of a very large number of base-station antennas has beenadvocated for some time, which is commonly referred to as “massive MIMO”and widely considered as one of the leading candidate technologies for5^(th) Generation (5G) cellular networks. In the present disclosure, theterm many-antenna is used to refer to a base station that has many moreantennas relative to a number of users the base station serves.

For some embodiments, efficient channel estimation in many-antennaMU-MIMO wireless communication systems (e.g., the MU-MIMO wirelesscommunication system 100 illustrated in FIG. 1) may require uplinkpilots that are used to infer the downlink CSI via Time Division Duplex(TDD) reciprocity. Since channel estimates may be only ephemerallyaccurate, downlink beamforming may need to occur immediately afterchannel estimation. As a result, an efficient many-antenna MU-MIMOtransmission frame structure may require several distinctive parts,i.e., beamsweeping, CSI collection, downlink communication and uplinkcommunication, as illustrated in FIGS. 6A, 6B, 6C, and 6D and discussedin more detail below.

Control Channel Operations

In wireless communication systems (e.g., the many-antenna MU-MIMOwireless communication system 100 illustrated in FIG. 1), communicationover a control channel may be employed to perform operations required tosetup data communication. Operations performed over the control channelmay include: synchronization, gain control, association, timing advance,random access, paging, setting modulation rates, gain control,scheduling, and so on. Additionally, the control channel may coordinateefficient CST collection across many antennas from multiple users.Described embodiments support control channel operations required toestablish an MU-MIMO channel, i.e., synchronization, association, CSIcollection, random access, and paging. Remaining control channeloperations may be performed over the established MU-MIMO channel.

Since nodes (e.g., the mobile users 104 shown in FIG. 1) in wirelessnetworks do not share oscillators, their time-frequency reference issubject to drift. Thus, all high-performance digital wirelesscommunication schemes require time-frequency synchronization. The users104 may establish time-frequency synchronization based on severaloperations. First, a user (e.g., any mobile user 104 shown in FIG. 1)may auto-correlate a received signal for frame detection and coarsetiming. Then, the user may perform automatic gain control (AGC) toensure the received signal is within an appropriate dynamic range of ananalog-to-digital converter (ADC) employed at the user's equipment.Next, the user may perform a cross-correlation with a pre-known sequenceto achieve fine-grained time synchronization. Finally, the user mayleverage a distortion within the known signal, i.e., phase shift, torecover a frequency offset and establish frequency synchronization.

For example, in 802.11-based wireless communication systems, a usercontinuously performs an auto-correlation to detect a short trainingsequence (STS) at a beginning of a packet, which triggers AGC. Then, theuser performs a cross-correlation on a following long training sequence(LTS) for time synchronization. Similarly, in Long Term Evolution (LTE)based wireless communication systems, a user continuously performs anauto-correlation to detect a cyclic prefix of each symbol. Then, theuser performs a cross-correlation on a Primary Synchronization Signal(PSS) and a Secondary Synchronization Signal (SSS) for timesynchronization. Typically, reference symbols can be transmittedthroughout a frame to maintain the synchronization, as well as tocompensate for other channel effects.

For certain embodiments, before a user (e.g., any of the mobile users104 shown in FIG. 1) can transmit or receive data, the user firstidentifies nearby base stations, selects one base station, and thenconnects to (i.e., associates with) that selected base station (e.g.,the base station 102 wireless communication systems in FIG. 1). Tofacilitate the association procedure, each base station transmits aunique identifier (e.g., a beacon) at a regular interval. Each userscans for base stations (e.g., for beacons transmitted from basestations), often over multiple frequencies, then chooses one basestation to associate with based on specific criteria, such as a signalstrength and authorization. Then, the user contacts the selected basestation, usually leveraging the same mechanism as random access, torequest and coordinate access, e.g., authorization, encryption, andscheduling.

To obtain CSI, a transmitter (e.g., the base station 102 illustrated inFIG. 1) may send a pre-known sequence (e.g., a pilot), which a receiver(a mobile user 104 shown in FIG. 1) may use to compute an amplitude andphase shift for each subcarrier channel. However, this approach requirestime-frequency synchronization, since without time synchronization thereceiver would not reliably know where the pilot starts, and withoutfrequency synchronization there would be an inter-subcarrierinterference that causes inaccurate channel estimation.

Traditional MU-MIMO wireless communication systems employ explicit CSIestimation, i.e., a base station may send pilots from each base-stationantenna, and users may estimate CSI to each base-station antenna andthen send the CSI estimation back to the base-station. For example, inCarrier Sense Multiple Access (CSMA) systems, CSI collection may beperformed at a beginning of every transmission frame, whereas inscheduled systems (e.g., LTE systems) CSI collection may be performedcontinuously using reference symbols from each base-station antenna.However, these techniques do not scale well as a number of antennas andusers increase. Because of that, emerging many-antenna systems typicallyemploy implicit CSI estimation, i.e., each user may send an uplink pilotthat a serving base station receives on every antenna, which provides anuplink CSI; the base station may then leverage reciprocal calibration toestimate a downlink CSI based on the known uplink CSI.

Additionally, the control channel of MU-MIMO wireless communicationsystem may handle notifying users when the users have incoming data,which is referred to as paging in the present disclosure. Furthermore, abase station may utilize the control channel to coordinate users torandomly access a network when the users have outgoing data, which isreferred to as random access in the present disclosure. Both paging andrandom access may need to occur before CSI is acquired, because a userneeds to be paged before sending pilots and the user needs to notify thebase station that it has outgoing data so the base station knows toestimate a channel associated with the user.

Gain Gap Between CSI and No-CSI Modes

Many-antenna base stations can operate in two modes: with knowledge ofCSI (e.g., in CSI mode) or without knowledge of CSI (e.g., in no-CSImode). FIG. 2 illustrates an example wireless communication system 200where a many-antenna base station 202 can operate in different modes, inaccordance with embodiments of the present disclosure. With CSIknowledge (e.g., in the CSI mode), the many-antenna base station 202 canachieve a gain of M²(where M is a number of base-station antennas)relative to a peak-power of a single antenna, which is illustrated by acoverage region 204. The largest gain can be achieved in the CSI modewhen the many-antenna base station 202 utilizes closed-loop beamformingwhen communicating to a single user, which is illustrated by a coverageregion 206.

On the other hand, without CSI knowledge (e.g., in the no-CSI mode), themany-antenna base station 202 may only have a gain of one for somecontrol channel operations, which is illustrated by a smaller coverageregion 208. Hence, a significant gain gap exits when operating the basestation in the CSI mode and in the no-CSI mode. Furthermore, while awireless communication channel can be reciprocal for uplink and downlinktransmissions, a transceiver hardware is not (e.g., a transceiverhardware at the many-antenna base station 202), which subsequentlycreates another gain gap between uplink and downlink communicationmodes.

By employing open-loop beamforming in the no-CSI mode, a coverage region(i.e., gain) per user is shown in FIG. 2 as a beam pattern space region210. In some embodiments, as discussed in more detail below, a coverageregion (i.e., gain) can be increased by applying coding gain on top ofopen-loop beamforming, leading to an extended coverage region per user,which is shown in FIG. 2 as a beam pattern space region 212.

FIG. 3 is an example table 300 showing analytical results of gains andgain gaps for different modes of operation of a many-antenna basestation (e.g., the base station 202 illustrated in FIG. 2), inaccordance with embodiments of the present disclosure. Table 300summarizes the analytical results for no-CSI and CSI modes of operationsfor both downlink and uplink communications when an M antenna basestation (e.g., the many-antenna base station 202 illustrated in FIG. 2)serves K single-antenna users. In one or more embodiments, eachbase-station antenna has a (peak) transmit power of P_(BS) and each userantenna has a (peak) transmit power of P_(U). For simplicity, averagechannel and antenna gains are normalized to 1, since they are constantacross all modes of operation, and include any non-reciprocal hardwareeffects, such as gains from low-noise amplifiers (LNAs) in anappropriate transmit power, e.g., the peak transmit power for each userantenna P_(U) includes the gain from the base station's LNAs.

It should be noted that there is no existing scheme which performsbetter than a single antenna for the no-C SI mode control channeloperations of synchronization and channel estimation. Thus, the no-CSImode has a gain of 1, which becomes P_(BS) and P_(U) for downlink anduplink, respectively, as shown in the table 300 in FIG. 3. The gain of Mantenna base station in the no-CSI mode can be dependent on whatoperation the base station is performing. For example, for CSIcollection, there is a fundamental gain limitation of 1 because CSI onlycomprises information about a link between one antenna and anotherantenna. Therefore, signals received at other antennas do not compriseinformation about that link's CSI. On the other hand, this theoreticallimitation does not exist for synchronization, as a desired signal canbe transmitted from all base-station antennas, which is exploitedherein.

While there are no-CSI mode techniques that achieve a theoretic gain ofM, these methods are either impractical, or, in fact, reduce theperformance of time-frequency synchronization. One naive approach wouldbe to use a radio frequency (RF) combiner to merge the power output ofthe M base-station antennas to a single antenna. However, this isdifficult and expensive to implement in hardware, as it requires perfectphase matching to avoid feedback to the antennas and complex wiring.Furthermore, by applying this approach, the diversity gain of M antennasis lost since only a single high-power transmitter is effectivelyemployed, i.e., a system is no longer M×K system. Another approach canbe to apply cyclic delay diversity (CDD), which cyclically rotatessymbols by different amounts of time from each antenna. The CDD spreadsthe power output of all M antennas spatially, and can be considered asarbitrarily beamforming on different subcarriers. However, the CDDcauses time-domain distortion, which substantially degrades theperformance of existing synchronization techniques. Furthermore, theperformance of CDD degrades rapidly as more antennas are added. Itshould be also noted that both of the aforementioned approaches may onlyprovide a certain gain in a downlink, and do not provide any gain in anuplink.

It is well known that the potential power gain of an M×K MU-MIMO systemwith CSI, in both uplink and downlink, is equal to P·M, where P is atransmission power. Leveraging CSI, a base station of the MU-MIMOwireless communication system can direct radiation towards, or listen toradiation from, intended K users using beams with an approximate widthof 1/M, which provides a spatial power gain of M. In the downlink, thebase station transmits power from all M antennas, but splits the poweramong K users, thus providing a per-link power of P_(BS)·M/K, for equalpower allocation among the users. In the uplink, the base stationreceives power from each user on all M antennas, thus providing aper-link power of P_(U). This renders a total gain of M²·P_(BS)/K forthe downlink and M·P_(U) for the uplink, as shown in the table 300 inFIG. 3. It should be noted that a MU-MIMO base station capable ofserving K users likely will not always serve K users simultaneously;with a single user the gap between the CSI mode and the no-C SI mode forthe downlink increases to a full M².

Control Channel Design for Gain Matching

Described embodiments include methods to bridge the aforementioned gaingaps by designing a control channel that overcomes limitations of no-CSIoperational mode. To bridge the gain gap between the no-CSI mode ofoperation and CSI mode of operation in the downlink, the control channeldesign presented herein may combine open-loop beamforming with a codinggain. For some embodiments, a many-antenna base station of amany-antenna MU-MIMO wireless communication system (e.g., themany-antenna base station 202 illustrated in FIG. 2) may sweep open-loopbeams carrying orthogonal sequences, which enable synchronization andpaging operations. In the uplink, the control channel design presentedin this disclosure exploits the natural per-antenna asymmetric transmitpower and employs an additional coding gain to enable CSI collection andrandom access operations. Furthermore, by encoding a base-station ID inthe downlink synchronization sequence and exploiting the random accessoperation, the control channel design presented herein facilitates theassociation operation.

Open-Loop Beamforming

In some embodiments, open-loop beamforming may be employed over thecontrol channel in the no-CSI mode to exploit the power and diversity ofall antennas at a many-antenna base station (e.g., the base station 202illustrated in FIG. 2). The combined power of the base station antennasprovides a gain of M, whereas the beamforming provides another gain ofM, for a total gain of O(M²). However, the beamforming gain focuses theradiated power on 1/M of the antennas' coverage area. Thus, themany-antenna base station (e.g., the many-antenna base station 202illustrated in FIG. 2) must sweep beams to provide complete coverage.Since the association and synchronization are delay-tolerant, thecontrol channel design employs open-loop beamforming and beamsweepingfor the association and synchronization operations without impactinguser-perceived performance or creating significant channel overhead.

While there are many MIMO and diversity schemes that exploit the gainsfrom multiple antennas, only open-loop beamforming can be effective fortime-frequency synchronization, as it provides the full potentialcombined power and directivity gain from all of the available antennaswithout causing time-domain distortion. Furthermore, open-loopbeamforming may have several practical benefits in MU-MIMO wirelesscommunication systems. First, the increased received power may allow auser to employ cheaper RF components, e.g., the LNA. Second, theincreased directivity and lower total power may reduce the interferenceto adjacent cells. Third, the open-loop beamforming does not require anyadditional hardware or computation, as the beamforming precoders arealready required to be applied at a many-antenna base station forMU-MIMO communication. Fourth, the open-loop beamforming allows thecoverage area to be finely tuned.

FIG. 4 illustrates an example 400 of a many-antenna base station 402that performs open-loop beamforming for users synchronization, inaccordance with embodiments of the present disclosure. To overcome thespatial selectivity of open-loop beamforming, the many-antenna basestation 402 may employ beamsweeping that transmits a signal, s, indifferent spatial directions using beamforming. Fundamentally,beamsweeping trades off increased spatial coverage with additional timeoverhead. Since, as discussed above, some control channel operations aredelay-tolerant, beamsweeping can be applied for synchronization, as wellas to facilitate association.

For some embodiments, each beam (e.g., beam 404 illustrated in FIG. 4)can be defined by a M×1 vector, b_(n), wherein an N length sweep patterncan be defined by a M×N matrix, B, composed of b₁,b₂, . . . , b_(n)vectors. The M-antenna base station 402 may transmit an entire sweeppattern in N time-slots, as the transmission in a given time-slot n andgiven base station antenna m is s·B_(m,n). Thus, if each beam 404 issent contiguously, the beamsweeping takes N times longer than a singleomnidirectional transmission of the same sequence. In one or moreembodiments, the many-antenna base station 402 may send a beam at abeginning of each frame. In this case, the entire beamsweep may have aduration of N·F, where F is the frame duration.

If the M×N matrix B forms an orthogonal basis, i.e., the matrix Bconsists of N=M orthogonal or pseudo-orthogonal beams, then completespatial coverage may be provided. In one or more embodiments, anycomplete M-dimensional basis used for beamsweeping may provide completecoverage of the CSI space, since, by definition, the CSI of any user canbe represented by a linear combination of the basis. This ensures that,for any given point in the coverage area, at least one beam in B willnot have a perfect null.

It should be noted that as the number of base station antennas Mincreases, the probability that a user detects a given beam is reduced,since the energy is more spatially selective. However, the probabilitythat a user will detect at least one beam in the sweep patternincreases, as, given a complete orthogonal basis, at least one beam ispointed towards the user, wherein that beam has a higher EffectiveIsotropic Radiated Power (EIRP) since the beam is narrower.

The control channel design presented herein that can be employed inmany-antenna MU-MIMO systems can leverage many beamforming techniqueswith compelling tradeoffs for specific implementations. Without detailedinformation about the environment and precise calibration, anyorthogonal basis with a low peak to average power ratio (PAPR) can besuitable for open-loop beamforming. While a complete basis guaranteesspatial coverage, it does not guarantee a strong signal. Since it isstatistically impossible that every user will have an open-loop beampointed directly at that user, the gain of beamsweeping may be reducedby an inaccuracy factor of a, i.e., to M²/a. As such, an overcomplete B,i.e., for N>M, can provide extended coverage by statistically reducingthe inaccuracy factor of a. Otherwise, given careful consideration ofthe propagation environment and antenna placement, as well as hardwarecalibration, techniques such as DFT open-loop beamforming can be tunedto provide a desired coverage area. In one or more embodiments, Hadamardbeamforming weights may be utilized.

In some embodiments, an order of beamsweeping can be selected such thata latency of reaching a user is reduced. For example, if thebeamsweeping order is continuously left to right (or vice versa), thenit may take a longer time on average for the beam to reach the user.However, if the beamsweeping is performed by hopping from one portion ofspace around a base station to another, i.e., beamsweeping is performedstarting from the left portion of space followed by the right portion,followed by the front portion and then the back portion, the averagelatency of reaching the user can be reduced.

In some embodiments, a coverage area can be increased when utilizingnon-repeating beamsweeping pattern. A many-antenna base station can beconfigured to continuously change a beam during the beamsweepingprocess. In this way, the many-antenna base station may cover more spacethan that when using a fixed beamsweeping pattern.

Coding Gain

For certain embodiments, the use of open-loop beamsweeping can reducethe gain gap between no-CSI and CSI modes. As illustrated in FIG. 4,gain of traditional no-CSI mode without beamsweeping is given by acoverage region 406, which is substantially smaller (e.g., by a factorof M²) than a coverage region (gain) 408 of MU-MIMO CSI mode. Byemploying the open-loop beamsweeping in no-CSI mode, the gain gap fromthe CSI mode can be substantially reduced. As illustrated in FIG. 4, acumulative gain (coverage region) of open-loop beamsweeping is given byinclusion of all beams of space 404 covering N different beamdirections. However, as shown in FIG. 4, there is still a coverage gap410 between the no-CSI open-loop beamsweeping and the CSI MU-MIMOcommunication.

To close the remaining gap between the no-CSI mode and the CSI mode, themany-antenna MU-MIMO system of the present disclosure additionallyemploys, in the no-CSI mode, a variable coding gain in both the downlinkand uplink communications. In some embodiments, a coding gain can beachieved by sending a signal over a longer period of time, thus, a totalreceived power, integrated over time, may increase linearly as theduration increases. However, the coding gain may come at a cost oflinearly increasing a channel usage overhead. For some embodiments,coding gains are preferred methods for tuning the gains to match betweenoperation modes because the coding gains are adjustable and thus can beused to dynamically fine-tune the gain vs. overhead tradeoff.

FIG. 5 illustrates an example 500 of a many-antenna base station 502that performs open-loop beamforming with applying additional codinggain, in accordance with embodiments of the present disclosure. Asillustrated in FIG. 5, by applying the coding gain in the downlink, acoverage region (i.e., gain) per beam may be increased from a coverageregion 504 (e.g., the coverage region when only the open-loopbeamsweeping is applied) to a coverage region 506 (e.g., the coverageregion when coding gain is combined with open-loop beamsweeping). Thus,by combining the open-loop beamsweeping with the coding gain, thecoverage gap 410 illustrated in FIG. 4 may be completely eliminated, andthe full coverage can be achieved in the downlink.

Referring back to FIG. 3, while table 300 analyzes the gain gap in termsof signal-to-interference-plus-noise ratio (SINR), it should be notedthat not all parts of the frame have the same SINR requirements. Forexample, data transfer can benefit from a higher SINR by altering themodulation and coding scheme. Higher-order modulation requires a higherSINR to be successfully decoded, thus the higher-order modulation can beconsidered as a negative coding gain in the CSI mode. For example, in802.11-based systems, Orthogonal Frequency Division Multiplexing (OFDM)binary phase shift keying (BPSK) modulation may require 15 dB SINR,whereas 64-QAM may require 31 dB. In contrast, the detection thresholdfor a length 128 Kasami sequence is approximately −5 dB. Thiseffectively further reduces the gain gap between the CSI mode used fordata transmission and no-CSI mode, but the gain gap reduction isdependent on actual data modulation rate. By leveraging a dynamic codinggain, the range and overhead of the many-antenna MU-MIMO system of thepresent disclosure can be tuned to the specific needs of eachdeployment.

FIGS. 6A, 6B, 6C, and 6D illustrate examples of different framestructures for different operations of a many-antenna MU-MIMO wirelesscommunication system, in accordance with embodiments of the presentdisclosure. For some embodiments, as illustrated in FIG. 6A, amany-antenna base station 602 may transmit (e.g., using a downlinkcontrol channel) to a user 604A variable length orthogonalsynchronization sequences (e.g., beacons 606) that may also encode thebase-station ID. As further illustrated in FIG. 6A and discussed in moredetail below, the many-antenna base station 602 may transmit (e.g.,simultaneously with beacon) a paging sequence 608 for paging anotheruser 604B different than the user 604A that performs synchronizationwith the many-antenna base station 602.

The many-antenna base station 602 may simultaneously providesynchronization and achieve a gain, C_(down), proportional to the lengthof the synchronization sequence. Since the synchronization sequencesneed to be detected prior to synchronization, the synchronizationsequences require low streaming auto-correlations, both with themselvesand the other sequences in the orthogonal set. That is, since thesynchronization sequences must be detectable without knowledge of whenthey start, a receiver (e.g., the user 604A in FIG. 6A) may need toperform a full correlation at every sample. Thus, a time-shift of thesynchronization sequences may need to produce a low correlation;otherwise it may cause an erroneous detection.

For some embodiments, after synchronization as illustrated in FIG. 6B,the many-antenna base station 602 may assign orthogonal pilot slots610A, 610B to active users 604A, 604B, 604C, etc., and may reservededicated uplink pilot slots 612 for association and random access(e.g., CSI collection). In one or more embodiments, the uplink pilotslots may be of variable length to enable a coding gain based on users'channel quality, e.g., users on cell edges may utilize longer pilots toincrease the accuracy of channel estimation. For some embodiments, asillustrated in FIG. 6C and FIG. 6D, the many-antenna base station 602may leverage the acquired CSI to provide downlink and uplinkconnectivity to users 604A, 604B, 604C, etc., as well as any remainingcontrol channel information over the efficient MU-MIMO communicationlink.

For some embodiments, by orthogonalizing pilots in frequency, thecontrol channel design is able to increase the accuracy of channelestimation, and provide an uplink gain of at least K. Frequencyorthogonalization (e.g., Orthogonal Frequency-Division Multiple Access(OFDMA)) may enable all users to transmit simultaneously, whichincreases the instantaneous power received at the many-antenna basestation by a factor of K. To collect complete CSI for every frequency,users can be further time orthogonalized, as shown in FIG. 6B, i.e.,orthogonal pilots 610A, 610B may be allocated to different users 604A,604B, 604C, etc. and simultaneously transmitted in uplink. As such, thetotal power received for a given user, integrated over time, alsoincreases by a factor of K. To obtain accurate CSI, each user may berequired to send a pilot for at least a duration of the inverse of thefrequency coherence, every coherence time interval. However, byscheduling users with poor channel quality to send even longer thanrequired by the frequency coherence interval, the coding gain, C_(up)may be increased. This approach ensures high-quality channelmeasurements across the entire cell and fully closes the gain gap, asillustrated in FIG. 5.

In one or more embodiments, for association and random access, users maysend orthogonal synchronization sequences on dedicated time-frequencyblocks during the training phase. This may allow the users to stillachieve a coding gain, while simultaneously enabling collision avoidanceand timing-advance estimation, as discussed in more detail below.

Combined Gain

For some embodiments, as discussed, combination of open-loop beamformingand coding gain may be employed over a control channel in many-antennaMU-MIMO system to close the gain gap, as illustrated in FIG. 5. In oneor more embodiments, beamsweeping may provide the majority of downlinkgain by focusing the full power of a many-antenna base station on asmall portion of the coverage area, i.e., the open-loop beamsweeping mayachieve a gain of M²/a, where a is the beamforming inaccuracy. In thedownlink, the control channel design reduces the gap between no-CSI andCSI modes of operation from M²/K to M²/K/(C_(down)·M²/a)=a/(C_(down)·K),thus the coding gain can be tuned so that C_(down)≈a/K. In the uplink,the control channel design leverages OFDMA and coding to achieve a gainof C_(up)·K in the no-CSI mode. This reduces the no-CSI to CSI gap fromM to M/(K·C_(up)), which suggests C_(up) should be approximately M/K toclose the gap.

However, once a proper downlink coding gain, C_(down), is applied,combined with open-loop beamsweeping, the no-CSI downlink gain is M²/K.In contrast, the no-CSI uplink gain is only (C_(up)·K·P_(U)), whichleads to a new gain gap. To mitigate the uplink-downlink gap, the totaltransmission power of the base station and user need to be approximatelythe same, e.g., O(P_(U))≈O(M·P_(BS)); this is typical of existingbidirectional communication systems, though macro cells can have as highas a 10 to 18 dB difference. This reduces the gap from(C_(up)·K·P_(U))/(M²/K·P_(BS)) to (C_(up)·K²)/M, and suggests that theuplink coding gain should be tuned to approximately M/K², along with anyresidual discrepancy between P_(U) and P_(BS), to finish closing thegap.

Comparing the uplink coding gain C_(up) needed for closing the gap inno-CSI vs. CSI, i.e., M/K, and the uplink coding gain C_(up) needed forclosing the gap in uplink vs. downlink, i.e., M/K², it can be observedthat there is a residual gap of K. Since the range of the base stationis limited by the downlink mode, C_(up) should be selected, for certainembodiments, to match the uplink-downlink gap. Then, the residual gainof K in the CSI uplink can be used to reduce transmission power orincrease modulation rate. In one or more embodiments, full coding gaincan be only required at cell edges, where users utilize extra-longpilots. It should be also noted that the many-antenna MU-MIMO systempresented herein, for a given coverage area, reduces the requiredper-antenna transmission power of the base station by M² and of the userby K.

Control Channel Design

Described embodiments relate to a control channel design and the usageof control channel for synchronization, association, CSI collection,random access, and paging, as will be discussed in more detail below.

Synchronization

The many-antenna MU-MIMO system presented herein achieves both timesynchronization and frequency synchronization. In some embodiments, thesynchronization can be achieved based on extended-length sequencestransmitted from a many-antenna base station to a mobile user byemploying beamsweeping.

FIG. 7 illustrates an example 700 of a many-antenna base station 702transmitting a synchronization sequence by employing beamsweeping, inaccordance with embodiments of the present disclosure. As illustrated inFIG. 7, the many-antenna base station 702 may be configured to transmitthe synchronization sequence using N different beam directions 704. Forsome embodiments, users may perform a streaming cross-correlation onreceived samples to detect the synchronization sequence sent from thebase station 702. Each user may compute the correlation of the receivedsignal R with the sequence S, i.e., Σ_(t=1) ^(n)(R_(t−1)·S*_(i)), atevery sample. The correlation may produce a peak at the single samplewhen R and S are aligned in time, i.e., time-synchronized.

The control channel design presented herein faces two main challenges.First, multiple synchronization sequences may need to be detectedsimultaneously since both beacon and paging sequences may be used forsynchronization, which are sent simultaneously on separate beams.Second, time synchronization needs to be performed without coarse timinginformation or AGC. As discussed above, coarse frame detection and AGCmay be employed in the CSI mode to achieve fine-grain timesynchronization. However, these techniques are inefficient or evenimpossible to employ in the no-CSI mode since the beamsweeps and MU-MIMOdownlink are highly spatially selective and, therefore, users receiveevery synchronization sequence with highly varying power. Themany-antenna base station 702 may be configured to precede transmissionof every synchronization sequence with transmitting a training sequenceto facilitate coarse frame detection and AGC. However, the trainingsequence may need to have significantly increased length to overcome thegain gap. Moreover, the gains set by this training sequence would onlybe valid for a single beam, making it highly inefficient forbeamsweeping.

Described embodiments address the aforementioned challenges based onthree techniques discussed in more detail herein. Firstly, for someembodiments, two full-precision correlators may be employed forperforming the streaming cross-correlation on received samples. Byperforming two parallel full-precision correlations, e.g., 12-bitcorrelations, the many-antenna MU-MIMO system presented herein mayreliably detect synchronization sequences with highly varying signalstrengths, as well as reliably distinguish paging and beaconsynchronization sequences that are sent simultaneously.

Secondly, for some embodiments, since performing AGC on every sequenceis inefficient, transmit gain control may be employed. Since themany-antenna base station 702 beamsweeps the synchronization sequence, auser receives every sequence with a substantially different signalstrength. Therefore, users may wait for a sequence in the sweep that iswithin the users' dynamic range. If the users do not detect anysequences, e.g., before discovering any base stations, the users mayslowly vary their receive gain settings until they detect sequences. Thegain settings can be modified such that to increase a number ofsynchronization sequences and beacons falling within the users' dynamicrange. After synchronization is established, the users may listen to allof the subsequent synchronization sequences and adjust their gainaccordingly. In one or more embodiments, the many-antenna MU-MIMO systempresented herein performs uplink gain control by using feedback, whereasfine-grain downlink gain control may be performed at the beginning ofeach downlink phase, as illustrated in FIG. 6C and FIG. 6D with segments614, 616.

Thirdly, for some embodiments, a detection threshold may be setdynamically by combining a running average of the correlator output anda spike detector. This is because, without traditional AGC, thesingle-sample correlation peak may vary drastically in magnitude. Theaverage correlator output may provide the average input power, but maybe additionally scaled by the power of the correlation sequence so thatdifferent sequences can be detected without adjusting the detectionthreshold. In one or more embodiments, the spike detector may simplyraise the detection threshold exponentially when there is a short burstof power, thus avoiding erroneous false-positives.

For some embodiments, to determine a carrier frequency offset (CFO), auser may calculate a phase drift in the received downlinksynchronization sequence. In one or more embodiments, the downlinksynchronization sequence may comprise two repetitions of the samesub-sequence; since the drift from CFO is constant, correspondingreceived samples in each repetition have the same phase offset. That is,for an n length sub-sequence repeated twice to form the synchronizationsequence S, θ(S_(i),S_(i+n))=θ(S_(j),S_(j+n)), where θ is the phasedifference between a pair of complex samples the synchronizationsequence S. This is because S_(i) and S_(i+n) are complex samples of thesame symbol. Thus, in the absence of CFO, θ(S_(i),S_(i+n))=0. With CFO,there is a phase drift that is proportional to time n, which is thusconstant across all complex samples i, i.e., θ(S_(i),S_(i+n))=drift(n).Therefore, CFO may be computed as:

$\begin{matrix}{{CFO} = {\frac{1}{2{\pi \cdot n}}{\sum\limits_{i = 1}^{n}{{\theta \left( {S_{i},S_{i + n}} \right)}.}}}} & (1)\end{matrix}$

In one or more embodiments, the division by 2π indicated in equation (1)is not performed since the CFO is multiplied by 2π when generating thecorrecting complex sinusoid. Thus, by selecting n to be a power of 2,the division in equation (1) becomes a simple bit-shift operation. In anembodiment, in the presence of noise, longer synchronization sequencesmay become more reliable, as the noise can be filtered out by theaveraging operation. The technique for frequency synchronizationpresented herein enables two synchronization sequences to besimultaneously transmitted (e.g., during beamsweeping) without affectingCFO recovery. Since both simultaneously transmitted synchronizationsequences comprise sub-sequences that repeat twice, the combined signalalso repeats twice and can still be used to accurately calculate CFO. Insome embodiments, if there is no other sequence being sentsimultaneously with the synchronization sequence, CFO can be calculatedwithout employing the repetitions of the synchronization sequence.

To avoid frequency distortion in multipath environments, a cyclic prefixmay be prepended to the synchronization sequence. However, the prependedcyclic prefix may make time synchronization less robust, as the cyclicprefix can cause false positives in the correlator, since the cyclicprefix aligns with a subset of the sequence. To avoid this, a cyclicpostfix may be employed, but then the CFO calculation may be delayedaccordingly, i.e., the sum in equation (1) may start at a length of thecyclic postfix. It should be noted that this approach does not affectthe correlator performance, as the correlator operates in thetime-domain.

Association Procedure

The presented many-antenna MU-MIMO wireless system enables associationby: (i) encoding a unique base-station identifier (e.g., beacon) in thebeamswept synchronization sequence, (ii) having users scan for theencoded beacons to select a base station, and (iii) providing a “soft”association mechanism that allows users to quickly obtain moreinformation about the selected base station over a MIMO link. Moredetails about each operation are provided herein.

For some embodiments, every base station may beamsweep a synchronizationsequence that encodes a locally unique identifier, called a beacon, asillustrated in FIG. 6A and discussed above. This approach may enableusers to simultaneously synchronize with a base station, as well as toidentify the base station. For the sake of brevity, the base stationsare considered to be coordinated so that they each have locally uniqueidentifiers and can ensure that their beacons do not overlap in time,which prevents random access collisions and reduces pilot contamination.

For some embodiments, before associating, a user may listen for at leastone entire sweep interval (possibly on multiple frequencies) todetermine the IDs of all nearby base stations, as well as the averagepower of the beacons from each base station. Since the beacon isbeamformed, its received power does not indicate an actual channelquality between the user and the base station. Thus, the user may needto listen to beacons for an entire sweep interval to obtain a roughestimate of the signal strength from each base station. However, thetrue SINR and channel quality cannot be accurately determined untilafter association due to the beamforming inaccuracy discussed above.Furthermore, the unique identifier contained in the beacon may notconvey any additional information, such as authentication, encryption,and a human-readable identifier (e.g., a Service Set Identifier (SSID)).Therefore, in or more embodiments, the user may be configured tosoft-associate to multiple base stations to search for the best match.

Since the beacons implemented herein may only contain a uniqueidentifier, the additional mechanism called soft-association may beprovided that enables users to gather more information over the CSImode. Traditional control channel designs broadcast information about abase station within beacons. For example, 802.11-based beacons mayinclude the Basic Service Set Identifier (BSSID), SSID, modulation rate,encryption information, and the like. This information can be utilizedby each user to determine if the user wants to, or even can, connect tothe base station. Moreover, the user may need to be able to judge itschannel quality to the base station, which can only be performed in theCSI mode.

For some embodiments, the soft-association mechanism implemented hereinmay enable users to quickly and efficiently establish a MIMO link withthe base station to efficiently exchange control channel information. Toperform the soft-association, each user may need first to synchronizewith the base station by successfully decoding a beacon. After that, theuser may send a pilot in one of the slots reserved for random access, asdiscussed in more detail below. Once the base station successfullyreceives the pilot, the base station has information about CSI for thatuser, and may use the CSI information to open a MIMO link and convey theremaining control channel information to the user. If the user proceedswith a full association (e.g., based on authorization, link quality, andthe like), the base station may schedule user-dedicated pilot slots anda unique paging sequence to maintain the link with the user. Otherwise,the user may continue to scan for and soft-associate to other basestations in the neighborhood before associating with only one basestation.

Collecting CSI

For some embodiments, after beacon detection, all active users may senduplink pilots in their scheduled slots, as illustrated in FIG. 6B. Then,the base station may utilize the received uplink pilots to collect CSIrelated to channels between the base station and the users. The CSIcollection phase may comprise a number of time-frequency-code resourceslots that can be arbitrarily assigned to users, with some resourceslots dedicated to random access (e.g., slot 612 illustrated in FIG.6B), including association requests and paging responses. For supportingthe association requests and paging, the random access slot 612 can bedivided between an association slot and random access. In one or moreembodiments, users that send reference signals in a given resourceelement may gain spatial resource elements in the corresponding time andfrequency coherence interval for both the uplink and downlink phases.Based on this, any given reference symbol may provide an estimation thatis valid both for the coherence time interval, as well as for a widerfrequency coherence interval. For certain embodiments, longer pilotslots may be assigned to users having quality of channels below acertain threshold to improve CSI accuracy.

Random Access

As illustrated in FIG. 6B, disclosed embodiments include a method forrandom access based on reserving pilot slots at the beginning of eachchannel estimation phase (e.g., the random access slot 612 illustratedin FIG. 6B). To initiate a connection with the many-antenna base station602, users 604A, 604B, 604C, etc. may send an uplink pilot in one ofthese reserved pilot slots. For the user 602 to send uplink pilotswithin correct pilot slots, without interfering with other users, theuser 602 may need to have successfully received a beacon, and thusestablished synchronization with the many-antenna base station 602. Inone or more embodiments, the base station 602 may utilize the receiveduplink pilot(s) to estimate the user's channel, as well as timingadvance, and create a highly efficient MU-MIMO link to the user. Thecreated MU-MIMO link may be then used to convey all remaining controlchannel information, including modulation rates and pilot scheduling, aswell as maintain/improve synchronization for active users if the beaconhas not been received for a pre-determined time period. In someembodiments, the conveyed control channel information may comprises atleast one of: BSSID, SSID, a modulation rate, gain control information,channel estimation information related to the MU-MIMO communicationlink, or encryption information associated with the many-antenna basestation.

LTE wireless communication standard specification provides thecompelling random access solution which can be suitable for themany-antenna MU-MIMO system presented herein, with the exception thatthe many-antenna MU-MIMO system presented herein allows for longerlength sequences to be employed to finely tune the gain gap. Asspecified by the LTE, the many-antenna MU-MIMO system presented hereinmay also employ, for random access, collision detection and avoidance,as well as timing advance.

Paging

Described embodiments enable a many-antenna base station of themany-antenna MU-MIMO system presented herein to reliably and quicklypage users across an entire coverage area of the many-antenna basestation. To accomplish this, the beamsweeping and coding gains describedabove can be applied. However, unlike synchronization and association,paging is not delay tolerant operation. Because of that, themany-antenna base station may utilize the users' last known location tosubstantially reduce the delay from beamsweeping.

FIG. 8 illustrates an example 800 of a many-antenna base station 802that performs paging of users 804A, 804B, 804C, in accordance withembodiments of the present disclosure. Upon association, the basestation 802 may assign each user a unique paging sequence. This pagingsequence may be constructed and transmitted almost identically to abeacon and simultaneously with the beacon, as illustrated in FIG. 6A. Asalso illustrated in FIG. 8, the many-antenna base station 802 maytransmit a beacon 806 for synchronization (e.g., using beamsweeping)simultaneously with transmitting paging sequences 808 to the users 804A,804B, 804C that are already synchronized and associated with themany-antenna base station 802.

For some embodiments, the paging sequence (e.g., the paging sequence808) may be chosen from the same codebook as the beacon (e.g., thebeacon 806) to ensure orthogonality. Furthermore, the paging sequencemay be repeated twice to facilitate time-frequency synchronization. Topage a user, the base station (e.g., the base station 602 illustrated inFIG. 6A, the base station 802 illustrated in FIG. 8) beamsweeps, alongwith a beacon (e.g., beacon 606 shown in FIG. 6A, beacon 806 shown inFIG. 8), a unique paging sequence (e.g., paging sequence 608 shown inFIG. 6A, paging sequence 808 shown in FIG. 8) associated with the userat the beginning of each frame, but on a separate beam, as illustratedin FIG. 6A and FIG. 8. This additional spatial separation between thebeacon and the paging sequence may improve the detection of either, asit reduces the inter-sequence interference. In one or more embodiments,to detect the paging sequence (e.g., paging sequence 608 in FIG. 6A,paging sequence 808 in FIG. 8), users (e.g., users 604A, 604B, 604C inFIGS. 6A, 6B, 6C, and 6D, users 804A, 804B, 804C in FIG. 8) may performthe same synchronization correlation used for the beacon (e.g., beacon606 in FIG. 6A, beacon 806 in FIG. 8), described above. Successfuldetection of the paging sequence similarly provides the user withsynchronization. However, in the case of detecting a paging sequence,the user may be configured to immediately send an uplink pilot in thepreviously dedicated (e.g., upon association) random access pilot slot.The transmission of uplink pilot may allow the base station (e.g., themany-antenna base station 802 in FIG. 8) to estimate CSI and begin MIMOcommunication with the paged user (e.g., the user 804A in FIG. 8).

In some embodiments, each paging sequence is transmitted to acorresponding user until the many-antenna base station 802 receives anacknowledgement from the user that the paging sequence is successfullyreceived. The beacon 806 and the paging sequences 808 can besuccessfully detected at the corresponding users as the beacon 806 andthe paging sequences 808 and their beam directions arepseudo-orthogonal. In an embodiment, the repetition of the beacon 806and the paging sequences 808 can be implemented to assist the users inrecovering CFO during overlapped paging and beacon sequences. In anotherembodiment, the beacon 806 and the paging sequences 808 can betransmitted at separate time frame(s). In this case, it is not requiredto repeat the beacon 806 and the paging sequences 808 to recover CFO atthe users, as a phase drift within the beacon sequence 806 and thepaging sequences 808 can be detected without interference.

While association and synchronization are not time-sensitive, the delayfrom beamsweeping may be unacceptable for paging. Therefore, in someembodiments, the many-antenna base station 802 may utilize the knowledgeof the user's prior location (e.g., last known location of the user804A) to guide the beamsweep, which can significantly speed up pagingoperation. It should be noted that leveraging the user's last knownlocation can only improve expected paging delay, as the sweep continuesuntil the user is paged. In some embodiments, the user's locationinformation may comprise at least one of: a physical location of theuser, CSI associated with the user, information about strengths of oneor more beacons received at the user (e.g., information about astrongest beam of the beacon sweep received at the user), an angle ofarrival of the strongest beacon received at the user, or any otherinformation that can facilitate steering the transmission beam from themany-antenna base station 802 to that particular user.

Additionally, or alternatively, the users 804A, 804B, 804C mayperiodically send a random access request (e.g., polling) to themany-antenna base station 802. This approach may serve multi-purpose ofmaintaining the association, checking for missed page requests, andupdating the users' last known location at the many-antenna base station802 to assist with efficient paging and inter-base station handovers.

Disclosed embodiments further include methods for optimizing the beaconsweep and paging search using historical user information for a givendeployment. For example, over time, a base station can learn that usersare never in certain deployment positions in space (e.g., up in thesky), and the base station can be configured to not sweep beams towardthese specific positions in space or to sweep the beams less frequently.Similarly, the base station can learn that users have typical movementpatterns. Thus, if a user does not respond to a paging sequence sentfrom the base station, the base station can be configured to perform thebeam sweep for sending the paging sequence towards anticipated userlocations instead of all possible directions. The anticipated userlocations can be based on at least one of: a last known physicallocation of a user, CSI associated with the user, information aboutstrengths of beacons received at the user, information about a strongestbeam of the beacon sweep received at the user, an angle of arrival ofthe strongest beacon received at the user, and the like.

Disclosed embodiments further include methods for synchronization andassociation of multi-antenna users during no-C SI operational mode ofthe many antenna base station. In some embodiments, each multi-antennauser can receive samples of a synchronization sequence on multiplebeamforming streams. Then, multiple correlations (e.g., streamingcross-correlations or autocorrelations) can be performed on the samplesof the synchronization sequence on the multiple beamforming streams todecode a beacon with an identification of the many-antenna base stationthat is encoded into the synchronization sequence. In some otherembodiments, a synchronization sequence can be detected on any antennaof a multi-antenna user based on an autocorrelation or cross-correlationat a low threshold. After that, beamforming weights can be computedbased on the detected synchronization sequence. A subset of the samplesof the synchronization sequence can be then processed based on streamingcross-correlations or autocorrelation on multiple user antennas usingthe computed beamforming weights to decode a beacon with anidentification of the many-antenna base station that is encoded into thesynchronization sequence. In some embodiments, the beamforming weightscan be pre-determined, and may comprise at least one of Hadamard-basedbeamforming weights or DFT-based beamforming weights.

Overhead Analysis

The control channel design presented in this disclosure may have asmall, if not negligible, overhead. For some embodiments, this overheadcan be measured by four metrics: (i) total channel overhead, (ii)association delay, (iii) random access delay, and (iv) paging delay.FIG. 9 provides equations 900 for determining these overheads anddelays. Table 910 given in FIG. 9 provides example values forillustrative system configurations. For this analysis, it can be assumedthat frames are transmitted continuously, with a beacon at the beginningof each frame. The expected paging delay is dependent on the pagingscheme. However, the expected paging delay is upper-bounded by theassociation delay, as that is how long it takes to perform a fullbeam-sweep.

For some embodiments, active users do not need to receive valid beaconsto maintain synchronization, as the synchronization can be maintained inthe CSI downlink control phase. Inactive, but associated users can alsomaintain synchronization by listening for beacons and paging signals.The duration that time-frequency synchronization is valid may depend onthe accuracy of the oscillators, frame design (e.g., cyclic prefix), aswell as fluctuations in temperature. Given the typical accuracy ofoscillators, the synchronization can be valid for hundreds of ms, butthis can be determined on a per-system basis. As such, beacons may beonly needed for association, and thus the sweep interval can be adjustedaccordingly. In one or more embodiments, the overheads shown in FIG. 9can be tuned by changing the system parameters. It should be note that,per table 910 in FIG. 9, the control channel design presented in thisdisclosure can support thousands of antennas with less than 2% overhead,at the cost of slightly increased association delay at the cell edges.

Implementation

In accordance with some illustrative embodiments, the control channeldesign presented herein may be implemented on a prototype of amany-antenna MU-MIMO base station that comprises an array of 108antennas, although more antennas may be also supported. The illustrativeembodiments employ Hadamard beamweights for beamsweeping. The Hadamardbeamweights use a minimal number of weights to provide a complete,perfectly orthogonal, basis, which may enable a full diversity gain andprovide complete spatial coverage with the minimal amount of overhead.Further, the Hadamard beamweights may feature a preferredpeak-to-average power ratio (PAPR) of 1, which may allow the basestation antennas to use their full potential transmit power.Additionally, calculating the Hadamard beamweights does not require anyknowledge of the antenna aperture or environment, enabling rapiddeployment without calibration or environmental considerations.

The illustrative embodiments utilize Kasami sequences for the downlinkcoding. Kasami sequences may provide desired detection performance, andmay have low, bounded, streaming correlation both with themselves andother orthogonal sequences. This allows the Kasami sequences to bereliably detected without time synchronization. Moreover, the Kasamisequences may provide a large number of orthogonal sequences, e.g., 4096for a length 256 Kasami sequence, which enables co-located users andbase stations to be uniquely identified.

The illustrative embodiments use Zadoff-Chu sequences for the uplinkchannel estimation coding. The Zadoff-Chu sequences have a constantamplitude and thus have a preferred level of PAPR. Furthermore, theZadoff-Chu sequences can be used to detect multiple users' random accessrequests simultaneously, along with each users' path delay to estimatetiming advance, with small computational overhead. Variable lengthZadoff-Chu sequences may be employed herein to match gain requirements,as well as for CSI estimation.

The illustrative embodiments support a real-time streaming time-domaincorrelator for the beacon, paging, and synchronization, which creates avery strong single-sample peak when the correct sequence is detected. Assuch, the performance range and accuracy is highly dependent on thedetection threshold. Since gain control for the beacon or paging code isnot performed in the present disclosure, the detection threshold is setdynamically based on the input power. In addition, the detectionthreshold may increase during power surges to avoid false-positives.Furthermore, the dynamic detection threshold can be scaled by aconstant, which may be controlled by a computer-programmable softwarecode. The dynamic detection threshold can be further optimized toincrease range, particularly with mechanisms to avoid false positives.

Performance Results

The performance of control channel design presented herein formany-antenna MU-MIMO systems are evaluated regarding synchronization,beacons, and paging in diverse environments for bridging the gain gapbetween the CSI mode and no-CSI mode of operation. The results presentedherein demonstrate that the presented control channel design can extendthe no-CSI mode range by over 40 dB when compared to traditional controlchannels. Furthermore, by leveraging knowledge of the user's previouslocation paging delay can be improved by 400%, and CFO of over 10 kHzcan be reliably corrected.

The performance of control channel design presented in this disclosureare tested in 100 discrete user locations at varying distances from thebase station in indoor environments and an anechoic chamber. Due tohardware availability, and for testing the performance of differentantennas, the presented control channel design is employed with threeseparate antenna configurations: (i) in the anechoic chamber with 80directional 6 dBi patch antennas, (ii) indoors and outdoors with 104omnidirectional 3 dRi monopole antennas, and (iii) indoors with 108omnidirectional 3 dBi monopole antennas. In all configurations the usersalso leveraged the 3 dBi omnidirectional antennas (e.g., one antenna peruser).

At each location, the control channel system presented herein is testedover a 20 MHz bandwidth at 2.4 GHz and the performance are analyzed withregard to the accurate detection of the beacon, paging signal, anduplink pilot, which demonstrate performance of the control channeldesign in the no-CSI mode. As a control, an unbeamformed beacon andpaging signal are additionally sent from each base-station antenna,i.e., “beamsweep” is performed by applying the identity matrix, in bothlow and high-power modes using a 64 length code to compare theperformance with traditional single antenna systems and the naivehigh-power solution discussed above. While the implemented controlchannel design is capable of running in real time, the implementationbriefly pauses after every beam to collect performance statistics fromthe nodes, such as successful detections, false positives, and receivedsignal strength indicators (RSSIs). Because of this measurement delay,the experiments are conducted without mobility, in relatively stationarychannels. The obtained results are used to analyze the performance ofthe presented control channel design beacon, paging, and CSI collectionvs. traditional methods. Additionally, a controlled experiment is setupto test the performance of the CFO estimator.

FIGS. 10 and 11 show the probability of successfully receiving the basestation's beacon, i.e., the synchronization sequence encoded with thebase-station ID, with various configuration parameters. In FIGS. 10-11,single-antenna transmission, both high power (e.g., bars 1002 in FIG.10, plot 1102 in FIG. 11) and low power (e.g., bars 1004 in FIG. 10,plot 1104 in FIG. 11), can be compared with diversity transmission(e.g., bars 1006 in FIG. 10, plot 1106 in FIG. 11) and the presentedcontrol channel design with code lengths of 64 (e.g., bars 1008 in FIG.10, plot 1108 in FIG. 11) and 128 (e.g., bars 1010 in FIG. 10, plot 1110in FIG. 11). In the case of single antenna diversity mode, the basestation rotates which antenna is transmitting, thus exploiting the fulldiversity of the array. This approach is equivalent to the presentedcontrol channel design using the identity matrix for beamsweeping.

FIGS. 10 and 11 sort the results based on the average uplink CSI signalstrength across all base-station antennas for the given location, whichis an approximation of distance and a fair metric for coverage area. Itshould be noted that downlink RSSI is not a good metric, since it variesper-beam. In addition, distance is not a good metric since scattererscan significantly alter signal strength. Clearly, changing uplinktransmission power will simply shift the same plots shown in FIG. 11either left or right, which indicates how code length and both uplinkand downlink transmission powers should be balanced in a real system.

The results across all locations are illustrated in FIG. 10, withseparate bars for the 36 anechoic chamber locations (left bars) and 64indoor locations (right bars), including 104-antenna locations and108-antenna locations. It can be observed in FIG. 10 that in indoorlocations the presented system (e.g., left bars 1010, 1008) is able toreliably serve significantly more locations than the traditional controlchannel (e.g., left bar 1006 for diversity scheme) and a single highpower antenna system (e.g., left bar 1002). Even when users have over a−70 dBm average RSSI to the base station, they miss almost 25% of thebeacons sent with the high-power single-antenna scheme (e.g., see leftbars 1004). This is due to multipath; in some locations, even fairlyclose, two paths can destructively interfere and create a null, which isnot easily overcome with additional signal strength. While the diversityscheme (e.g., see bars 1006) performs better than the single antenna(e.g., see bars 1002), the diversity scheme is still unable to reliablyreceive many beacons where users have lower than −70 dBm uplink RSSI.This illustrates the necessity of the control channel design presentedin this disclosure, which leverages both the power and diversity of theentire array, in many-antenna MU-MIMO systems (e.g., see beacondetection performance denoted with bars 1008 and 1010 in FIG. 10).

FIG. 11 illustrates beacon detection performance results from theanechoic chamber. Since there is no multipath in the anechoic chamber,the detection rate of each technique is very closely related to RSSI,thus these results accurately demonstrate the relative performance ofeach technique. It can be observed from FIG. 11 that the presentedcontrol channel design (e.g., plots 1108 and 1110) is able to outperforma single-antenna scheme (e.g., plot 1102) by over 40 dB, and thehigh-power scheme (e.g., plot 1104) by 20 dB.

To demonstrate the ability of the presented control channel design toleverage location information to accelerate paging, a simple scheme istested where the paging sweep is guided based on the intended user'slast location. The experiments are performed on the 108-antenna basestation configuration in the last 44 locations. Mobile users are pagedbased on each beam's detectability, which is determined by thecorrelation magnitude to threshold ratio.

It is determined that the base state employing the presented controlchannel design is able to successfully page 94% of users by the secondframe, compared to only 70% without leveraging the user location, asillustrated in FIG. 12. When users are near the base station, the usersreceive the majority of the beams in a sweep, and thus optimizing basedon the users' location does not provide much benefit, as shown by thelow RSSI plot 1202. However, the paging delay is reduced from an averageof 4.8 frames to 1.2 frames, an improvement of 4 fold, and a worst-caseimprovement of 68 frames to 3 frames (plots 1202 and 1204 vs. thepresented control channel design illustrated by plot 1206).

While successful detection of a beacon or paging sequence inherentlyprovides time-frequency synchronization, to more accurately test theaccuracy of the presented CFO correction, a more controlled experimentis setup herein. A reference clock is shared between the base stationand user, effectively removing CFO, and the user is placed at 0.5 m fromthe base station. Then, a controlled CFO is induced in the beaconsequence by multiplying it with a complex sinusoid ranging from −10 kHzto 10 kHz. To measure the performance vs. coding gain and SNR, beaconsof length 64 and 128 are sent, and attenuators are used at the basestation to reduce the transmission power from −12 dBm to −42 dBm. Theseattenuations resulted in the user receiving roughly −60 dBm (High), −75dBm (Mid), and −90 dBm (Low) RSSIs. The cumulative distribution of theerror magnitude of the CFO estimates is presented in FIG. 13. Forclarity, the results presented in FIG. 13 are derived from a singleestimation; however multiple estimates can be employed to reduce theerror by an order of magnitude.

It can be observed from FIG. 13 that with mid and high RSSI thepresented system is always able to correct CFO within 0.8 kHz using a128-length beacon (e.g., plots 1302, 1304), and within 1.3 kHz using a64-length beacon (e.g., plots 1308, 1310). In the low RSSI regime, itcan be observed that the 64-length beacon (e.g., plot 1312) begins toperform poorly, and is only able to correct 80% of the beacons to within2 kHz error. In contrast, the 128-length beacon with low RSSI (e.g.,plot 1306) performs similarly to the high RSSI 64-length (e.g., plot1308), which indicates that extending the beacon length can furtherreduce CFO estimation error. It should be also noted that the amount ofinduced CFO does not affect accuracy.

FIG. 14 illustrates various components that may be utilized in awireless device 1402 that may be employed within the system 100illustrated in FIG. 1, the system 200 illustrated in FIG. 2, the system500 illustrated in FIG. 5, the system illustrated in FIGS. 6A, 6B, 60C,and 6D, the system 700 illustrated in FIG. 7, and/or the system 800illustrated in FIG. 8. The wireless device 1402 is an example of adevice that may be configured to implement the various methods describedherein. The wireless device 1402 may be a many-antenna base station(e.g., the base station 102 in FIG. 1, the base station 202 in FIG. 2,the base station 502 in FIG. 5, the base station 602 in FIGS. 6A, 6B,6C, and 6D, the base station 702 in FIG. 7, and/or the base station 802in FIG. 8), or a user (access) terminal (e.g., the user terminal 104 inFIG. 1, the user terminal 604A, 604B, 604C, etc. in FIGS. 6A, 6B, 6C,and 6D, and/or the user terminals 804A, 804B, 804C in FIG. 8).

The wireless device 1402 may include a processor 1404 which controlsoperation of the wireless device 1402. The processor 1404 may also bereferred to as a central processing unit (CPU). Memory 1406, which mayinclude both read-only memory (ROM) and random access memory (RAM),provides instructions and data to the processor 1404. A portion of thememory 1406 may also include non-volatile random access memory (NVRAM).The processor 1404 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 1406. Theinstructions in the memory 1406 may be executable to implement themethods described herein.

The wireless device 1402 may also include a housing 1408 that mayinclude a transmitter 1410 and a receiver 1412 to allow transmission andreception of data between the wireless device 1402 and another wirelessnode (e.g., another wireless node in a remote location). The transmitter1410 and receiver 1412 may be combined into a transceiver 1414. One ormore antennas 1416 may be attached to the housing 1408 and electricallycoupled to the transceiver 1414. The wireless device 1402 may alsoinclude (not shown) multiple transmitters, multiple receivers, andmultiple transceivers.

The wireless device 1402 may also include a signal detector 1418 thatmay detect and quantify the level of signals received by the transceiver1414. The signal detector 1418 may quantify detection of such signalsusing total energy, energy per subcarrier per symbol, power spectraldensity and/or other quantification metrics. The wireless device 1402may also include a digital signal processor (DSP) 1420 for use inprocessing signals.

The various components of the wireless device 1402 may be coupled by abus system 1422, which may include a power bus, a control signal bus,and a status signal bus in addition to a data bus.

FIG. 15 is flow chart illustrating a method 1500 that may be performedat a many-antenna base station (e.g., the base station 502 illustratedin FIG. 5, and/or the base station 802 illustrated in FIG. 8), inaccordance with embodiments of the present disclosure.

Operations of the method 1500 may begin by an encoder of the manyantenna base station (e.g., the processor 1404 of the wireless device1402 illustrated in FIG. 14)) encodes 1502 a beacon (e.g., the beacon606 illustrated in FIG. 6A) into a base synchronization sequence (e.g.,synchronization sequence s), the beacon comprising an identifier of thebase station.

A circuit of the many-antenna base station (e.g., the processor 1404 ofthe wireless device 1402 illustrated in FIG. 14) generates 1504 aplurality of synchronization sequences based on the encoded basesynchronization sequence (e.g., sequences) and a set of beamformingweights (e.g., beamforming weights in sequences b₁,b₂, . . . , b_(n)),i.e., by covering (e.g., spreading) the encoded base synchronizationsequence (e.g., sequence s) with the sequences b₁,b₂, . . . , b_(n) withthe beamforming weights.

A transmitter of the many antenna base station (e.g., the transmitter1410 of the wireless device 1402 illustrated in FIG. 14) transmits 1506the plurality of synchronization sequences, using a plurality ofantennas, in a plurality of beam directions (e.g., beam directions 506illustrated in FIG. 5) associated with the set of beamforming weights.In an embodiment, the spatial beam directions are orthogonal to eachother. In another embodiment, the spatial beam directions are notorthogonal and a number of beam directions can be increased or decreasedin order to alter a desired coverage area. The beamforming weights canbe selected such that to limit the coverage area to only selected areas.In some embodiments, to achieve coding gain in the no-CSI mode, theplurality of synchronization sequences can be encoded based on at leastone of Gold codes, Kasami codes, or Zadoff-Chu codes before thetransmission.

FIG. 16 is a flow chart illustrating a method 1600 that may be performedat a user equipment (UE, such as UE 604A illustrated in FIG. 6A, UE 804Aillustrated in FIG. 8) in communication with a many-antenna base station(e.g., the base station 502 illustrated in FIG. 5, the base station 802illustrated in FIG. 8), in accordance with embodiments of the presentdisclosure.

Operations of the method 1600 may begin by a receiver of UE (e.g., thereceiver 1412 of the wireless device 1402 illustrated in FIG. 14)receives 1602 a plurality of synchronization sequences (e.g., sequencesR) having different signal strengths.

A first circuit of UE (e.g., the processor 1404 or DSP 1420 of thewireless device 1402 illustrated in FIG. 14) correlates 1604 samples ofa synchronization sequence from the plurality of synchronizationsequences with a set of identification sequences (e.g., a set ofpre-known base station identification sequences or beacons) to detecttiming of the synchronization sequence (i.e., to determine timesynchronization). In some embodiments, the UE may first perform anautocorrelation on the received samples of the synchronization sequenceto detect the existence of a repeating sequence within thesynchronization sequence and reduce the computational overhead ofstreaming cross-correlation by exploiting the existence of the repeatingsequence.

A second circuit of UE (e.g., the processor 1404 of the wireless device1402 illustrated in FIG. 14) decodes 1606, from the synchronizationsequence, a beacon (e.g., beacon 606 illustrated in FIG. 6A) having anidentifier of the many-antenna base station encoded in thesynchronization sequence.

Described embodiments include methods for designing an efficient controlchannel in many-antenna MU-MIMO wireless communication systems. Thepresented methods for control channel design provide fine-grainedcontrol over time, coding gains, and spatial resources, enablingoptimizations both within a base station and across a wirelesscommunication network. The presented control channel design allows basestations to leverage existing information, such as users' last knownlocation, traffic patterns, and environmental properties tointelligently optimize timing, coding gains, and spatial coverage.Moreover, these same properties can be used to further extend the rangeof the cell in sparse networks, restrict coverage area, carefully tuneinterference, or dynamically incorporate more antennas to increase thecapacity of a given base station.

Disclosed embodiments provide design, implementation, and experimentalvalidation of a wireless control channel in many-antenna MU-MIMOsystems. By holistically considering the practical design constraints ofmany-antenna base stations, a flexible design can be achieved thatimproves the range, or transmission efficiency, by over 40 dB on a 108antenna base station with negligible overhead. The presented controlchannel design provides flexible optimization of space, time, code, andfrequency resources, enabling it to scale from a few antennas up to1000s of antennas. Not only does the presented control channel designdrastically improve the performance of basic control channel operationsby leveraging MU-MIMO as much as possible, but it also utilizes spatialinformation to make paging operations as quick and efficient aspossible.

The various operations of methods described above may be performed byany suitable means capable of performing the corresponding functions.The means may include various hardware and/or software component(s)and/or module(s), including, but not limited to a circuit, anapplication specific integrated circuit (ASIC), or processor.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

The various operations of methods described above may be performed byany suitable means capable of performing the operations, such as varioushardware and/or software component(s), circuits, and/or module(s).Generally, any operations illustrated in the Figures may be performed bycorresponding functional means capable of performing the operations.

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Disclosed embodiments may also relate to an apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, and/or it may comprise a general-purpose computingdevice selectively activated or reconfigured by a computer programstored in the computer. Such a computer program may be stored in anon-transitory, tangible computer readable storage medium, or any typeof media suitable for storing electronic instructions, which may becoupled to a computer system bus. Furthermore, any computing systemsreferred to in the specification may include a single processor or maybe architectures employing multiple processor designs for increasedcomputing capability.

Disclosed embodiments may also relate to a product that is produced by acomputing process described herein. Such a product may compriseinformation resulting from a computing process, where the information isstored on a non-transitory, tangible computer readable storage mediumand may include any embodiment of a computer program product or otherdata combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

1. A method for optimizing wireless communications by a base station(BS), wherein said wireless communication exhibits 2 modes of operation,a no-CSI (open-loop) mode and a CSI (MIMO) mode occurring before the BShas CSI knowledge of supported active users and the CSI (MIMO) modeoccurs after time-frequency synchronization and collection of channelstate information (CSI), comprising: generating a plurality ofsynchronization sequences; transmitting a single synchronizationsequences followed by at least one repetition of that synchronizationsequence at the beginning of each frame; encoding a geographicallylocally unique identifier to the BS into said synchronization sequences;generating a set of beamforming weights allowing for spatial directionaltransmission of beam patterns within specific regions; altering beampatterns by changing beam forming weights applied to each antenna of theBS to modify the amplitude and phase of the signal sent from said BSantenna; transmitting, using a plurality of antenna, said beacon sent ina plurality of beam directions associated with said beam formingweights; receiving, by user equipment (UE), said geographically locallyunique ID of the BS via beacons; simultaneously identifying andsynchronizing with said BS through beacon decoding; transmitting anuplink pilot from the UE; estimating channel state information (CSI)associated with the UE based on the received pilot to the BS; collectingUE specific CSI information from said uplink pilot; establishing a CSImode (MIMO communication) link between said BS and said user based onknown CSI; using known CSI information between the many antenna BS anduser to calculate the beamweights that maximize signal strength at theintended user and minimize destructive interference of intended users;receiving at said BS location information and/or pilots of the user; andsteering beams of data toward said UE based on the received locationinformation and/or pilots.
 2. The method according to claim 1 whereinsaid UE soft-associates with one of a number of base stations (BS) inorder to exchange control channel information based on signal strengthand authorization to request coordinate access including authorization,encryption and scheduling.
 3. The method of claim 2 wherein said UE'ssoft-association occurs after a successful decoding of said beaconwhereby a UE may send a pilot to a BS and, once received by the BS, saidBS may utilize the UE CSI information to open a MIMO link.
 4. The methodof claim 3 wherein, if the UE proceeds to full association, said BS mayschedule user-dedicated pilot slots and a unique paging sequence tomaintain a link with the UE.
 5. The method of claim 4 wherein each userunique paging sequence may be constructed and transmitted simultaneouswith the beacon to users synchronized and associated with said BS in theCSI mode and transmitted, using the plurality of antennas, said pagingsequence in a set of beam directions until being received by that UE. 6.The method of claim 5 wherein time-frequency synchronization andassociation are delay tolerant and occurs via no-C SI (open-loop)beamforming and beamsweeping for synchronization and associationoperations and paging is not delay intolerant and occurs via CSI(closed-loop) beamforming.
 7. The method of claim 6 wherein the BS mayuse the knowledge of UE's location information associated with the UE,the location information comprises at least one of a physical locationof the UE, channel state information (C SI) associated with the UE,information about strengths of one or more of the synchronizationsequences received at the UE, or an angle of arrival of at least one ofthe synchronization sequences received at the UE to selectively guidebeemsweeps to accelerate time sensitive paging operations.
 8. The methodof claim 6 wherein beacon sweeps and paging search efficiencies areenhanced through the BS's use of historical spatial user data includinguser's last known physical location, traffic patterns, environmentalproperties, occupiable and non-occupiable space and typical movementpatterns to (1) direct beams toward a user's anticipated locationinstead of all possible directions, and/or (2) decrease beam frequency,and/or (3) omit specific positions in space.
 9. The method of claim 6wherein base stations leverage existing information, including spatialinformation, in the form of last known location, traffic patterns andenvironmental properties to optimize timing, coding gains, and spatialcoverage and to further extend the range of the cell in sparse networks,restrict coverage area, carefully tune interference, and/or dynamicallyincorporate more antennas to increase the capacity of a given basestation.
 10. The method of claim 6 wherein a BS can utilize userlocation based on last known physical location, CSI associated withuser, information about strengths of beacons received at the user,information about strongest beam of the beacon sweep received at theuser and an angle of arrival of the strongest beacon received at theuser to anticipate a user's location and decrease paging times.
 11. Themethod of claim 1, wherein, once a valid communication link in CSI modeis established, that communication link is used to convey all remainingcontrol channel information including at least one of the following:BBSSID, SSID, modulation rates, pilot and data scheduling, gain control,channel estimation information related to the communication link in CSImode, encryption information associated with the many-antenna basestation and synchronization for active users.
 12. The method of claim 6wherein users periodically send random access requests to the basestation to maintain synchronization and association, check for missedpaging requests, and to update a user's location to assist with amaintained, updated, or optimized control channel operation for thetransmittal of future paging sequences.
 13. The method of claim 1wherein a coding gain is employed in the no-CSI mode in both the uplinkand downlink communication extends a signal over a longer period of timethus tuning the gains to match between no-CSI mode and CSI mode,dynamically, balancing gain versus overhead.
 14. The method of claim 1wherein said two modes of operation, no-CSI and CSI modes, are utilizedto push time-critical information into the more efficient CSI mode anduse spatial information, current, historic and anticipated, to makepaging operations faster and more efficient.
 15. The method of claim 1wherein communication parameters including beam patterns, sweep rate andsynchronization length may be dynamically configured to match a requiredgain for full coverage of an area.
 16. The method of claim 1 wherein therange of the many-antenna BS may be extended by increasing open-loopbeamforming and coding gains in the no-CSI mode and reducing themodulation rate and UEs in the CSI mode.